This invention relates to a method of automatically detecting a state of surface treatment of a base plate such as etching in the production of photomask and, in particular, to a method of automatically detecting an end point of the treatment.
The photomask used for the production of semiconductors is generally produced by a known method comprising a process of applying photoresist to the base plate of glass or the like to the surface of which some metal is applied by means of vapor deposition, a process of making said photoresist film into a pattern by exposing a mask pattern onto the photoresist film and then developing the same using an applicable developing solution, and a process of etching the metallic film exposed by the development.
A high degree of accuracy is required particularly in the process of etching of all others in the production of photomask, and accordingly it is indispensable to accurately control the end point of the etching process. Thus, several attempts have been proposed for detecting the end point to a high degree of accuracy. According to the Japanese Patent Application laid open under Provisional Publication No. Sho 56-158872, for example, a method is proposed in which a region for measuring light transmittance is provided outside an effective area on the surface of the photomask to detect the progress of etching process and the light transmittance of said region being measured to detect the end point of etching based on the measured result.
Said prior invention, however, as a method of detecting the end point of etching process, employs an averaging method wherein the difference between the sampling values taken before and after the transmittance of light respectively, is averaged at the consecutive 10 points or so. Hence there is a problem that when the progress of etching is temporally stopped, for example, in the event of remaining a refractory oxide film on the metallic film formed by means of vapor deposition, it is likely to erroneously take said temporal stoppage for the end point before coming to the actual end point of the etching process. Also there exists another disadvantage that since the quantity of transmitted light on the base plate of glass to which a metallic film is applied by means of vapor deposition is variable depending upon the kind of metal, existence of some oxide film thereon, the characteristic of the glass base plate, the thickness thereof, etc., an accurate detection of the end point of the etching process is almost impossible so far as using said averaging method.
Further, in order to hold the glass base plate on the rotational treatment device (spinner) during the surface treatment, it is known to adopt either a vacuum suction device making use of a vacuum chuck of the spinner head or a simple stationary arm holder which holds the four corners of the base plate.
When the spinner head of the simple stationary arm holder is used, there exists a disadvantage that the arm holding the four corners of glass base plate intermittently interrupts the light flux or beam of which transmitted quantity on the base plate must be detected, making it further difficult to accurately detect the end point of etching process so long as using such averaging method.